Manipulation of the number, size and distribution of particle assemblies within the astrocytic cell membrane is best done in vitro. Primary cultures of astrocytes from 7-day-old rats are usually fed every 4 days, a schedule maintaining the fusiform shape of these cells and a paucity of assemblies. If the culture medium is "conditioned" by not feeding for 7 to 10 days, the cells become more normally asteroid in shape and the number of assemblies increases. The stimulus for these changes is not meningeal. Co-cultivation with cells from the pia-arachnoid results in cells no different from those derived from brains stripped of their leptomeninx. Small neurons, detected immunocytochemically, survive for at least 4 weeks and may be the source of the "conditioning" factor. In order to see whether the assemblies can be rearranged at all, the drastic treatment of denaturation with guanidine and urea was performed. Both denaturants caused clumping of assemblies into large aggregates to which background particles also adhered. Cytochalasin B did not rearrange the assemblies but colchicine caused an augmentation in their number and an aggregation comparable to denaturation, an effect suggesting that the assemblies may be anchored to the cytoplasmic matrix.